1. Field of the Invention
The present invention relates to an optical unit contained in an optical symbol reader such as a bar code reader, and more particularly to an optical scanner for scanning an optical symbol such as a bar code by deflecting a laser beam repeatedly.
2. Description of the Related Art
One type of the optical symbol reader such as the bar code reader scans an optical symbol such as a bar code with a laser beam. In the optical symbol reader of this type, a laser beam emitted from a light source such as a semiconductor laser is deflected in one-dimensional or two-dimensional direction by an optical scanner repeatedly to scan an optical symbol (hereinafter referred to as a bar code). A reflected light from the bar code passes through a converging lens and is detected by a photo detector such as a photodiode to decode the bar code in accordance with an electric signal output from the photo detector.
The optical scanner using a polygon mirror (also called a rotating polygon mirror) or a mirror (also called a galvanometer mirror) is well-known. In the optical scanner using the mirror, a laser beam emitted from the light source is reflected from the mirror and directed toward the bar code, and the mirror (or its reflection face) is angularly oscillated at regular intervals to scan the bar code with the laser beam. The related-art structure for vibrating (hereinafter referred to as angularly oscillating) the mirror is made as follows.
(1) A leaf spring has one end fixed, and holds a mirror at the other free end. A permanent magnet is fixed on the mirror, and an electromagnetic coil is disposed near the permanent magnet. The electromagnetic coil is excited at regular periods to give a drive force via the permanent magnet to the mirror. The mirror is vibrated (angularly oscillated) due to the drive force from the electromagnetic coil and a restoring force of the leaf spring member. This leaf spring system was described in, for example, JP-A-7-254041 or JP-A-11-326805.
(2) Another structure uses a rotating mechanism with a rotation shaft and a bearing of the rotation shaft. The mirror and the permanent magnet are fixed to a rotating member (oscillating member). A magnetic substance is disposed near the rotating member so as to exert a suction force between the magnetic substance and the permanent magnet. An electromagnetic coil is excited at regular periods to urge the permanent magnet in an opposite direction to the suction force. The mirror is vibrated (angularly oscillated) by a drive force from the electromagnetic coil and the suction force (acting as the restoring force).
The methods for driving the mirror or exciting the electromagnetic coil include a self-excitation and a separate excitation. The self-excitation method as used herein means a synchronous excitation with a resonance frequency determined by a moment of inertia of the rotating member (oscillating member) containing the mirror, and the restoring force. The separate excitation method as used herein means compulsorily giving a drive force with an exciting current at a lower frequency than the resonance frequency. The self-excitation method has the advantage that there is less exciting current and the drive power efficiency is maximized, but the disadvantage that the resonance frequency is determined by the mechanical structure and the degree of freedom is low. The separate excitation method has the disadvantage that more drive power is consumed and the drive power efficiency is low, because the resonance phenomenon is not used, although the drive period (frequency) can be set up at will to some extent.
The above angular oscillation structure (1) for the mirror has the problem that the structure of a leaf spring holding portion is complex, and the leaf spring is easily damaged due to an impact upon dropping or vibration during transportation. To resolve this problem, a stopper mechanism for restricting the movement of the mirror held at the free end of the leaf spring beyond a normal range of angular oscillation may be additionally provided, or an elastic member for absorbing the impact when the mirror makes contact with the stopper may be disposed. Alternatively, the rotation member (oscillating member) containing the mirror and the permanent magnet may be disposed so that the center of gravity may be matched to the center of rotation, with the center of rotation fixed with a certain gap.
However, even if any improvement method is employed, the number of parts is increased or the structure becomes complex, resulting in the increased manufacturing cost. When the separate excitation method is employed, the mechanical strength may be raised to withstand the impact or vibration by designing the elastic modulus for the leaf spring at a high value, but there is the drawback that the drive power efficiency is so low as to consume more drive current, as previously described.
Also, the angular oscillation structure (2) as above described has the advantage that the structure is simple and strong enough to withstand the impact upon dropping or the vibration during transportation, but has the problem of producing a great loss due to a sliding friction between the shaft and the bearing. Furthermore, since there is a great dispersion in the characteristics for the magnetic substance and the permanent magnet for generating a magnetic force (suction force) as the restoring force, with significant variations in the characteristics upon temperature changes, it is difficult to employ the self-excitation method. That is, the separate excitation method with lower drive power efficiency is obliged to employ.
When lubricating oil is applied to reduce the sliding friction between the shaft and the bearing, dispersion in performance is more likely to occur due to the temperature characteristic of the viscosity of the lubricating oil. The suction force exerted between the magnetic substance and the permanent magnet is inversely proportional to the distance between them to the second power, the distance being changed nonlinearly, along with the rotation of permanent magnet (rotation member), depending on the arrangement of the magnetic substance and the permanent magnet. Accordingly, to obtain a smooth and stable angular oscillation of the mirror, a special drive current waveform may be required.